U.S. patent number 8,397,914 [Application Number 13/649,718] was granted by the patent office on 2013-03-19 for heat shrunk, manually openable flexible film package.
This patent grant is currently assigned to Curwood, Inc.. The grantee listed for this patent is Brian Joseph Conrad, Daniel Frank Janichek, Jr., Christopher Wayne Nimis, Matthew Christopher Peterka. Invention is credited to Brian Joseph Conrad, Daniel Frank Janichek, Jr., Christopher Wayne Nimis, Matthew Christopher Peterka.
United States Patent |
8,397,914 |
Janichek, Jr. , et
al. |
March 19, 2013 |
**Please see images for:
( Certificate of Correction ) ** |
Heat shrunk, manually openable flexible film package
Abstract
The present disclosure is concerned with heat shrunk sealed
packages of flexible transparent thermoplastic film that easily
opened by peeling. It involves lowering the peel strength of the
heat seal between the two films used to form the package by
providing one of them with a frangible layer by contaminating a
layer with an incompatible polymer and providing a set of graspable
tabs, one on each of the films, with which to initiate peeling of
the heat seal. The tabs are heat set so that they do not shrink
with the rest of the package. In a preferred embodiment, the
package is provided with a second heat seal just inside the
perimeter of the first and a stress concentrator, which may be a
chevron, at the juncture of heat set tabs which lie with one atop
the other with the first heat seal.
Inventors: |
Janichek, Jr.; Daniel Frank
(Neenah, WI), Conrad; Brian Joseph (Appleton, WI), Nimis;
Christopher Wayne (Oshkosh, WI), Peterka; Matthew
Christopher (Neenah, WI) |
Applicant: |
Name |
City |
State |
Country |
Type |
Janichek, Jr.; Daniel Frank
Conrad; Brian Joseph
Nimis; Christopher Wayne
Peterka; Matthew Christopher |
Neenah
Appleton
Oshkosh
Neenah |
WI
WI
WI
WI |
US
US
US
US |
|
|
Assignee: |
Curwood, Inc. (Oshkosh,
WI)
|
Family
ID: |
47844614 |
Appl.
No.: |
13/649,718 |
Filed: |
October 11, 2012 |
Current U.S.
Class: |
206/497; 426/415;
220/260 |
Current CPC
Class: |
B65D
77/2032 (20130101); B65D 77/2024 (20130101); B65D
77/2036 (20130101) |
Current International
Class: |
B65D
85/00 (20060101) |
Field of
Search: |
;206/497,557,564,524.8
;220/260 ;426/411,413,414,415 ;428/36.6,347 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Ackun; Jacob K
Attorney, Agent or Firm: Hall; Tom J.
Claims
What is claimed is:
1. A manually openable heat shrunk package comprising: a first
flexible heat shrinkable film; a second flexible heat shrinkable
film; a hermetic seal comprising a perimeter heat seal formed by
heat sealing a portion of said second flexible heat shrinkable film
to a portion of said first flexible heat shrinkable film; wherein
only one of said films is readily frangible and renders said heat
seal manually peelable; a heat set tab on each of said two flexible
heat shrinkable films which are aligned with each other; wherein
each of said tabs has a sufficient surface area facing the other
tab but not adhered to it to render said tab readily manually
graspable; wherein a portion of each of said tabs immediately
adjacent to said heat seal is adhered to the other and said adhered
portion includes a feature which concentrates stress on said heat
seal when said tabs are pulled away from each other in a direction
generally perpendicular to the plane of the heat seal; wherein said
graspable tabs are adapted to pull away from each other such that
said readily frangible film cleanly peels apart from itself to
rupture said heat seal and open said package.
2. The package of claim 1 wherein said feature is a chevron with
its point oriented away from said heat seal.
3. The package of claim 1 wherein said hermetic seal further
comprises a second heat seal formed adjacent to and entirely
surrounded by said perimeter heat seal; wherein said graspable tabs
are adapted to pull away from each other such that said readily
frangible film cleanly peels apart from itself to rupture said
perimeter heat seal and said second heat seal, and open said
package.
4. The package of claim 1 wherein both of said flexible films are
multilayer with the outer most layer of each adapted to be readily
heat sealable.
5. The package of claim 1 wherein said frangible film includes a
frangible layer.
6. The package of claim 5 wherein said frangible film further
includes a heat sealable layer.
7. The package of claim 6 wherein said frangible layer is
immediately adjacent to said heat sealable layer.
8. The package of claim 7 wherein the composition of said frangible
layer is adjusted such that it fractures adjacent to or at the
interface with said heat sealable layer upon said package being
opened.
9. The package of claim 6 wherein said frangible layer lies between
said heat sealable layer and a nylon layer and has a greater
adhesion to said heat sealable layer than to said nylon layer.
10. The package of claim 9 wherein said frangible layer is based on
an anhydride modified copolymer of ethylene which has been
contaminated with a polymer of butene-1 to reduce its adherence to
said nylon layer.
11. The package of claim 5 wherein said readily frangible layer is
rendered readily frangible by containing a polymer which is
incompatible with the polymer which provides the structure of said
readily frangible layer.
12. The package of claim 5 wherein said frangible layer is a tie
layer.
13. The package of claim 1 wherein said frangible film has an
initial peak peel strength in a standard ASTM F88 test using
300.degree. F. at 30 psi for one second is less than about 3200
g/inch.
14. The package of claim 13 wherein said initial peak peel strength
is between about 2400 and 3200 g/inch.
15. The package of claim 1 wherein said frangible film has a
propagation peel strength of between 150 grams/inch and 900
grams/inch.
16. The package of claim 15 wherein said propagation peel strength
is between 300 grams/inch and 900 grams/inch.
17. The package of claim 16 wherein said propagation peel strength
is between 500 grams/inch and 900 grams/inch.
18. The package of claim 17 wherein said propagation peel strength
is between 700 grams/inch and 900 grams/inch.
19. The package of claim 1 wherein both of said flexible films are
multilayer having an outer most heat sealable layer and at least
one of said heat sealable layers carries a printed area over a
portion thereof which forms one of said heat set tab.
20. The package of claim 19 wherein said printed area is sufficient
to prevent said area from adhering to the other flexible film when
said heat set tab is formed by applying heat to said portion while
it is in a face to face relationship with the other flexible film.
Description
SUMMARY OF THE INVENTION
The present invention is concerned with improved heat shrunk
packages formed from a first flexible heat shrinkable film and a
second flexible heat shrinkable film, and having a hermetic seal
comprising a heat seal formed by heat sealing a portion of the
second flexible heat shrinkable film to a portion of the first
flexible heat shrinkable film; wherein only one of the heat
shrinkable films is readily frangible and renders the heat seal
manually peelable. The inventive packages also include a heat set
tab on each of the two heat shrinkable films which are aligned with
each other such that each of the tabs has a sufficient surface area
facing the other tab but not adhered to it to render each tab
readily manually graspable. A portion of each of the tabs
immediately adjacent to the heat seal is adhered to the other and
the adhered portion includes a feature which concentrates stress on
the heat seal when the tabs are pulled away from each other in a
direction generally perpendicular to the plane of the heat seal.
The graspable tabs are adapted to pull away from each other such
that the readily frangible film cleanly peels apart from itself to
rupture the heat seal and open the package.
The tabs are conveniently provided by heat setting aligned portions
of the two films which lie outside the heat seal which joins them
to each other and provides the hermetic seal. These portions are
heat set by constrained annealing before the package is subjected
to shrinking in manner that avoids their adhering to each other and
are of a sufficient size as to be readily graspable. In a preferred
embodiment the surface of at least one of these two portions which
faces the other film is treated to render it non-adherent. One
approach is the print this surface with an ink and another is to
apply a label to this surface. Alternatively, a heat resistant
material which will not fuse to either of the facing surfaces may
be inserted between them or a heat set temperature may be utilized
which is below the temperature at which the two facing surfaces are
rendered adhesive.
The heat seal is conveniently rendered more amenable to peeling by
using two multilayer films where only one of the films is frangible
and provides at least one frangible layer, by which is meant a film
layer which has been designed to be particularly susceptible to
fracture. In a preferred embodiment the frangible layer of the
frangible film is adjacent to the exterior layer which participates
in the formation of the heat seal. In a particularly preferred
embodiment the frangible layer is formed by contaminating the
polymer which forms the structure of this layer with an
incompatible polymer. In a particularly preferred embodiment the
frangible layer is a tie layer.
In a preferred embodiment the hermetic seal is made more secure by
creating a second heat seal which is adjacent to and is entirely
surrounded by the first heat seal. The aim is to provide a means of
isolating the first heat seal from some of the stresses encountered
when the films are shrunk on to the packaged item or items.
In a preferred embodiment the first heat seal is rendered more
peelable by providing a stress concentrator immediately adjacent to
the first heat seal in the vicinity of where the two tabs join each
other and distal from the free ends of these tabs. In a
particularly preferred embodiment the stress concentrator is a
feature formed as an extension of the first heat seal located where
the tabs join one another. In an especially preferred embodiment
the stress concentrator is a chevron with its apex directed away
from the first heat seal. It is also possible to form the stress
concentrator by allowing a properly configured area of the two tabs
adjacent to the first heat seal to adhere or fuse together. The aim
is to lower the force needed to cause an initial fracture of the
first heat seal and thus facilitate the peeling apart of this heat
seal.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view of the molded container 10 showing the
projected corner 20 with its ink overprint 22 with its ink free
projected stress concentrator location 24, the sealing gasket 30
which facilitates the vacuum forming, the projected cut line 40
along which the container will be separated from the web stock (not
illustrated) out of which the container is thermoformed and the
interior of the container 50.
FIG. 2 is a plan view of the molded container 10 with the same
elements carrying the same numerical indicators.
FIG. 3 is a perspective view of the hest sealing apparatus 60
including an upper mold carrier 70 which has two seal bar carriers
80 which each carries seal bars 90 and a dog bone shaped movable
heatable platen 98.
FIG. 4A is a plan view of the heat sealing apparatus 60 with the
same elements carrying the same numerical indicators. Additionally
illustrated are the outer seal bar 92, the inner heat seal bar 94
and the stress concentrator profile 96.
FIG. 4B is a cross-section of a seal bar carrier 90 along line A-A
with the same elements carrying the same numerical indicators.
FIG. 5 is a perspective view of the sealed package 100 showing the
heat seals 102, the heat set tabs 104 and the stress concentrator
106 formed by the stress concentrator profile 96.
DETAILED DESCRIPTION OF THE INVENTION
The film package with which the present invention is concerned may
be conveniently manufactured by first forming the molded container
10 shown in FIGS. 1 and 2 by hot drawing constrained web stock into
a mold equipped with a gasket 30 to help maintain the vacuum used
to draw the web stock into a mold to form the interior 50. Later in
the process this molded container 10 will be separated from the web
stock from which it is formed along the projected cut line 40. This
will create projected corner 20. The portion of the web stock which
will become the projected corner 20 has been treated with an ink
overprint 22 such that when it is later brought into contact with
another web stock under heat and pressure it will not adhere to
said other web stock. However, the ink overprint has been omitted
from a triangularly shaped stress concentrator location 24 located
within the projected corner 20. The molded container is then filled
with the desired article, typically a meat or cheese product, by
placing it in the interior 50.
The open mouths of two side by side molded containers are sealed by
heat sealing a portion of a web stock caused to overlay these open
mouths using the seal bars 90 carried by the seal bar carriers 80
which are in turn carried by the upper mold carrier 70 which is the
operative portion of the heat sealing apparatus 60 as shown in
FIGS. 3 and 4A. The projected corner 20 of each molded container 10
and a corresponding portion of the overlaying web stock are heat
set by the overlaying web stock being contacted by the heatable
platen 98 and the heat transferring into the projected corners 20.
The ink overprint 22 prevents the corresponding portions of the
overlaying web stock from fusing or adhering to the projected
corners 20 despite their facing surfaces being forced together at
an elevated temperature.
As can be seen in FIG. 4A and FIG. 4B, the heat seal bars 90 are
made up of outer seal bars 92 and inner seal bars 94. As can be
seen in FIG. 4A the outer seal bars 92 each carry a stress
concentrator profile 96 which mates with the stress concentrator
location 24 and creates a stress concentrator 106 by fusing the
corresponding portion of the overlying web stock to the stress
concentrator location 24. In essence a triangular shaped projection
with its apex pointed outward is formed on each the heat seals
formed by the outer seal bars 92.
After the open mouth of the molded container 10 is sealed by the
action of the seal bars 90 to form the heat seals 102, the
projected corner 20 and the corresponding portion of the overlying
web stock have been heat set to form the heat set tabs 104 by the
action of the movable heatable platen 98 and the stress
concentrator 106 is formed by the action of the stress concentrator
profile 96, the sealed package is separated from the two web stocks
along the projected cut line 40 and subjected to elevated
temperature to cause heat shrinking of the web stock which has not
been heat set to yield the final easy open package illustrated in
FIG. 5. The heat sealing is effected in a sealed chamber such that
the interior of the molded container 50 is evacuated.
The films used to form the sealed package may be any thermoplastic
films which are adapted for use in making sealed heat shrunk
packages from cavities which are formed by hot stretching at least
one of the films. Preferably one or both of the films will be
essentially transparent. Both the films need to have an exterior
layer which is heat sealable using industry acceptable conditions.
It is preferred that they display an initial peak peel strength of
at least about 2000 g/inch when sealed at 300.degree. F. for 1
second at 30 psi. The term "initial peak peel strength" as used
throughout this disclosure refers to the amount of force required
to initiate the peeling or rupturing of a film from itself or to
another film. In contrast, the term "propagation peel strength" as
used throughout this disclosure refers to the amount of force
required to maintain the peeling or rupturing of a film from itself
or from another film after peeling has been initiated. In general,
the initial peak peel strength will be higher than the propagation
peel strength. Both of the films also need to have locked in
stresses from their manufacture so that they will exhibit
substantial shrinking when exposed to an appropriate elevated
temperature. It is preferred that they exhibit shrink in both the
machine direction (MD) and the transverse direction (TD) of at
least about 20% when exposed to 90.degree. C. It is particularly
preferred that they display shrinkages of about 35% or more when
exposed to 90.degree. C. The first film needs to exhibit
substantial stretchability at elevated temperatures. It is
preferred that it sustain an increase in area of about 300% or
greater at temperatures of about 80.degree. C. or greater. It is
convenient if the second film displays substantial stretchability
at elevated temperatures to accommodate the packaging of
particularly large items. It is preferred that both films be
approved for food contact. It is also preferred that both films
incorporate an oxygen barrier such as ethylene vinyl alcohol (EVOH)
or polyvinylidene chloride (PVdC) as one of its layers. It is also
preferred that the external layer opposite the heat seal layer be
puncture and abuse resistant. The nylons known for use in wrapping
and packaging films are particularly preferred for this purpose. An
especially preferred film structure is
Nylon/Tie/Nylon/EVOH/Nylon/Tie/Heat Sealable. Particularly suitable
nylon layers, which are made from mixtures of crystalline and
amorphous polyamides, are taught in U.S. Published Patent
Application 2008/0113130 incorporated by reference herein. The heat
sealable layer may be any of those conventionally employed to
obtain heat seals in flexible packaging films through the
application of elevated temperature and pressure. Layers based on
lower density polyethylene homopolymers such as low density
polyethylene (LDPE), very low density polyethylene (VLDPE) and
ultra low density polyethylene (ULDPE) and copolymers with small
amounts of .alpha.-olefins such as hexane or octane such as linear
low density polyethylene (LLDPE) and metallocene catalyzed linear
low density polyethylene (mLLDPE) by themselves or combined with
ethylene vinyl acetate (EVA) are preferred. Particularly preferred
are the heat seal layers taught by U.S. Pat. No. 8,039,070,
incorporated by reference herein. It is preferred that the heat
seal layer display a peak peel strength of at least about 3500
g/inch in ASTM F88 when sealed at 300.degree. F. at 30 psi for one
second.
One of the two films will have a frangible layer which promotes
peelability. This layer may be the heat seal layer, though it is
preferred that it be another layer, with an especial preference for
this layer being immediately adjacent to the heat seal layer. It is
preferred that the frangible layer be isolated from the heat seal
operation so that its frangibility or peelability is not unduly
effected by the heat sealing conditions and has fairly reproducible
peelability despite the variations in heat sealing parameters which
may be encountered across various packaging plants. This layer is
typically rendered frangible by contaminating the polymer or
polymers which make up its structure with an incompatible polymer.
It is preferred to use a polyethylene based structural polymer
contaminated with between about 10 and 40 wt. % polybutene.
Particularly preferred frangible layers are described in U.S. Pat.
No. 8,039,070. However, in the present invention it is preferred
that the frangible layer also be the tie layer between the heat
seal layer and the next adjacent layer rather than being interposed
between the heat seal layer and a tie layer. In the particularly
preferred overall film structure, there is a core EVOH oxygen
barrier layer sandwiched between two nylon layers and a tie layer
between the inner nylon layer and the heat seal layer. In this
construction it is preferred to contaminate this tie layer,
preferably with polybutene. It is preferred that this frangible
layer lower the initial peak peel strength to less than about 3200
g/inch in ASTM F88 when sealed at 300.degree. F. at 30 psi for one
second. It is particularly preferred that the initial peak seal
strength under these conditions be lowered to between about 2400
and 3200 g/inch by this frangible layer.
In the preferred film constructions there are one or more tie
layers which promote adhesion between layers that typically do not
adhere well to each other. These tie layers have a number of
formulations, some of which are disclosed in U.S. Pat. No.
8,039,070. But a typical characteristic is the inclusion an
anhydride modified polyethylene based polymer. In the case of tie
layers between nylon and a heat seal layer based on a lower density
polyethylene alone or combined with EVA, it is preferred to use
maleic anhydride modified polyethylene based copolymer which is
mostly ethylene, especially one with a low modulus. Particularly
preferred tie layers for such an application are those based on the
anhydride modified polyethylene copolymers disclosed in U.S. Pat.
No. 4,900,612, incorporated by reference herein. Especially
preferred are Equistar.RTM. PX 3080 anhydride modified linear low
density polyethylene resin having a melt index of 1.8 g/10 min.,
and a density of 0.910 g/cm.sup.3 which is supplied by Equistar
Chemicals, LP, Houston, Tex.; DuPont Bynel.RTM. 41E710 anhydride
modified linear low density polyethylene resin having a melt index
of 2.7 g/10 min, a density of 0.91 g/cm.sup.3, a melting point of
115.degree. C. and DuPont Bynel.RTM. 21E787 anhydride modified
ethylene acrylate resin having a melt index of 1.6 g/10 min., a
density of 0.93 g/cm.sup.3, a melting point of 92.degree. C.;
DuPont Bynel.RTM. 4164 anhydride modified linear low density
polyethylene resin having a melt index of 1.3 g/10 min., a density
of 0.93 g/cm.sup.3 and a melting point of 127.degree. C., all of
which are supplied by E. I. du Pont de Nemours and Company,
Wilmington, Del.; Mitsui ADMER.RTM. SF755A elastomer Based Adhesive
Resin, having a melt index of 2.6 g/10 min a density of 0.89
g/cm.sup.3 which is supplied by Mitsui Chemicals America, Inc., Rye
Brook, N.Y.; Westlake Tymax.TM. GT7058 anhydride modified acrylate
copolymer resin having a melt index of 2.70 g/10 min, a density of
0.943 g/cm.sup.3, and a melting point of 74.degree. C., Westlake
TYMAX.TM. GT4300 anhydride modified linear low density polyethylene
resin having a melt index of 8.0 g/10 min., a density of 0.918
g/cm.sup.3 and a melting point of 121.degree. C., both of which are
supplied by Westlake Chemical Corporation, Houston, Tex.; Dow
AMPLIFY.TM. GR 216 anhydride modified polyolefin elastomer resin
having a melt index of 1.3 g/10 min, a density of 0.875 g/cm.sup.3,
and a melting point of 62.8.degree. C. which is supplied by Dow
Chemical Company, Midland, Mich.
In the preferred construction in which the tie layer adjacent to
the heat seal layer is contaminated to render it frangible, it is
preferable to strike a balance so that its adhesion to the heat
seal layer is low enough to facilitate manual peeling for easy
opening but is sufficiently high enough to avoid skinning or
webbing. It is believed that skinning or webbing will be observed
if the adhesion of the tie layer is too low to reliably cause a
fracture of the heat seal layer. If the adhesion is too low to
cause a clean failure in the heat seal layer, on peeling the film
may simply delaminate leaving a membrane of the seal layer in place
(skinning) or pieces of the heat seal layer by be left behind
(webbing).
In the preferred constructions, the frangible film of the present
invention comprises a frangible tie layer. It is preferred that the
frangible film include a frangible tie layer such that the
frangible film has a propagation peel strength of between 150
grams/inch and 900 grams/inch, between 300 grams/inch and 900
grams/inch, between 500 grams/inch and 900 grams/inch, or between
700 grams/inch and 900 grams/inch.
It is preferred to use a tie layer which directs the fracture
caused by peeling to be adjacent to or at the interface with the
heat sealable layer. This is believed to minimize or eliminate the
occurrence of skinning or webbing upon opening of the sealed
package. One approach is to utilize a tie resin with a strong
affinity for the next interior layer and to contaminate it with an
additive which reduces its affinity with the heat sealable layer.
In the case of the preferred structure with a nylon interior layer
and a polyethylene based heat sealable layer, a preferable approach
is to use an anhydride modified copolymer of ethylene with butene-1
and to contaminate it with a polymer of mainly butene-1. This
contamination is believed to reduce the adhesion to the nylon layer
without having as great an effect on the adhesion to the heat
sealable layer.
It is also important that the frangible layer, whether a feature of
the heat seal layer or located adjacent to the heat seal layer,
have sufficient strength to survive heat shrinking onto the
contents of the sealed package. Typically for appearance purposes,
the cavity for accommodating the item or items to be packaged is
designed to cause the heat seal between the first film and the
second film to be immediately adjacent to these items. This means
that as the film is heat shrunk the heat seal will be subjected to
particularly high stresses. Such stresses can be sustained without
failure of the package by a careful adjustment of the frangible
layer to be in an appropriate window of strength.
In addition, an inner heat seal between the first and second films
which lies inside but adjacent to the outer heat seal and follows
the perimeter of the outer heat seal is preferred. It is believed
that upon heat shrinking this inner heat seal can absorb some of
the shrinking stresses and protect the outer heat seal from
failure. Such a seal is readily provided by using a seal bar which
carries two beads, one within the other. In order to give the
sealed package an optimum appearance, the outside seal should be
located fairly close to the packaged items, which then requires
that this inner heat seal be located fairly close to the outer one.
It is particularly preferred that the inner bead on the heat seal
bar be spaced less than about 5 mm from the outer bead. Because the
outer heat seal provides a hermetic seal to the package, it is not
necessary that the inner heat seal be continuous. However, it is
convenient and preferred that it be continuous.
The sealed package is provided with two heat set tabs, one formed
in each of the first and second films such that the one formed in
the top film lies under and is essentially coincident with the one
formed in the second film. The aim is to provide two graspable tabs
which can be used to pull the first film away from the second film
thereby peeling open the heat seals and allowing access to the
items packaged in the sealed package. It is desirable that these
tabs not undergo any significant shrinking when the sealed package
is subjected to heat shrinking conditions and consequently during
the formation of the sealed package they are subjected to elevated
temperatures while being constrained from shrinking. This can
conveniently be done by bringing a heated platen in contact with
the areas of the films destined to form the tabs at the same time
as the two films are heat sealed together. In the typical apparatus
the edges of the films are held by clips so these areas can not
shrink in the transverse direction and, at this point in
manufacture, the these areas are still part of a continuous web so
that they cannot shrink in the machine direction. However, it is
necessary to take some step to prevent the two facing surfaces of
these areas from adhering to each other in view of the fact that
they each carry the heat sealable layer of their film. One or both
of these surfaces may be treated to render it non-adherent, some
material may be placed between these surfaces such as a piece of
Teflon tape or the platen temperature may be constrained to be high
enough to effect the relaxation of shrinking stresses but too low
to effect any significant adhesion. It is preferred to print one of
these surfaces with an ink, although a label material conventional
in the industry may also be applied. It is particularly preferred
to pre-print the first film with an adhesion preventing ink in the
appropriate areas before feeding the roll stock of such film to the
package forming apparatus. Printing of the first film is preferred
because it is typically easier to ensure that the printed areas are
in registry with the areas destined to become the graspable tabs on
the first film than it is to obtain registry with the second film.
The ink may be any of those typically used to impart graphics to
such flexible packaging film. It is just necessary to apply a
sufficient thickness and this is readily determined by simple
trial.
The open mouth of typical mold used on this cavity forming
apparatus has a generally rectilinear shape which makes it
convenient to form the tabs on one of the corners of this shape. In
fact, it is common for these corners to have a radius which fits
within the right angle which would be formed by simply extending
the adjacent sides until they intersected. It is particularly
convenient to utilize the area between these right angles and these
radii for the formation of the tabs.
It is preferred to provide a stress concentrator or peel initiator
at the point where the tabs abut the outer heat seal. The stress
concentrator may be any feature which facilitates a fracture
through the heat seals when the two graspable tabs are pulled away
from each other in a direction generally perpendicular to the plane
of the heat seals. US Published Patent Application 2010-0270330,
incorporated by reference herein, discloses angular features which
are suitable as peel initiators. A particularly preferred stress
concentrator is a chevron or triangle with its apex directed way
from the heat seal bead and its base adjacent to this bead. This
stress concentrator is conveniently made by adding an appropriate
profile to the heat seal bead which forms the outer heat seal. For
instance a triangular shape could be added to this bead at one of
its radiused corners. Alternatively a stress concentrator could be
made by providing an unprotected shape on coinciding areas of the
facing surfaces of the two tabs and using a sufficiently high heat
set temperature. These two unprotected areas would then adhere
together to provide a feature of the appropriate shape. For
instance, the unprotected shape could be a triangle with its base
aligned to be adjacent to the outer heat seal.
The sealed package of the present invention may be generally made
in accordance with the teachings of U.S. Pat. No. 7,487,652,
incorporated by reference herein, with the addition of forming the
graspable heat set tabs and, in the preferred embodiments, the
stress concentrator and the inner heat seal. Another deviation from
the conventional practice is, of course, the use of a film with a
frangible layer. After heat sealing the first and second films
together, cutting this sealed portion from the webs of the two
films and heat shrinking the sealed package about the packaged
items by the application of elevated temperatures, a final package
is obtained which is hermetically sealed and stable to normal
shipping and handling but which can be readily opened by the
average person without the aid of any tool. He just needs to
manually pull the two graspable tabs away from each other in a
direction generally perpendicular to the plane of the heat
seals.
The sealed readily openable heat shrunk package of the present
invention may be used for the packaging of items which benefit from
being contained in form fitting hermetically sealed packages. In
those cases in which the films are multilayer and contain an oxygen
barrier, these packages are particularly advantageous for items of
perishable food items whose shelf life is enhanced by isolation
from the atmosphere and for whom there is a desire by consumers to
view the packaged items. Such thermoformed heat shrunk packages of
the present invention are particularly useful for meat and
cheese.
WORKING EXAMPLES
Example 1
A readily manually opened shrink wrapped package of a 32 ounce
turkey ham was made with an apparatus described in U.S. Pat. No.
7,487,625 modified to produce two aligned heat set tabs on the
upper and lower films used to form the package. Roll stock of a
transparent thermoplastic seven layer nylon based oxygen barrier
heat sealable 428 mm wide shrink film was fed to the apparatus. The
film had been made by a double bubble process and displayed shrink
performance of about 45% machine direction (MD) and 50% transverse
direction (TD) at 90.degree. C. One exterior layer was a heat
sealable mixture of a very low density polyethylene (VLDPE) with a
density of 0.91 g/cm.sup.3 and a melt flow of 0.6 with a
metallocene catalyzed linear low density polyethylene (mLLDPE) with
a density of 0.9 g/cm.sup.3 and a melt flow of 7.5 and the other
exterior layer was a mixture of crystalline and amorphous nylons
(crystalline resins with relative viscosities (RV) of about 3.6 and
about 1.8 and an amorphous resin with an intrinsic viscosity (IV)
of about 0.82). The core layer was ethylene vinyl alcohol
(EVOH).
This roll stock was passed over the top of two side by side molds
spaced apart 22 mm, each approximately 140 mm long by 175 mm wide
in the transfer direction of the web (the machine direction or MD)
with a depth of about 75 mm with the heat sealable exterior layer
facing up away from the mouth of the mold, heated to 85.degree. C.
for 1.5 seconds and stretched into the mold by the application of a
vacuum. The open mouths of the molds were provided with a gasket
which aided in maintaining the vacuum which was applied from below.
The edges of the web of roll stock were held by chain mounted clips
as is conventional in the operation of this apparatus. The chain
carrying the clips advanced with the web in the machine
direction.
The mold was then moved downward so that the web, now carrying two
open mouth cavities about 75 mm deep, could advance about 250 mm. A
32 ounce turkey ham was then placed into each cavity and the web
was advanced another 250 mm.
A web of roll stock of a second transparent thermoplastic seven
layer nylon based oxygen barrier heat sealable 428 mm wide shrink
film was passed over the open mouths of the cavities with its heat
sealable exterior layer facing toward the cavities. This film was
essentially identical in structure and manufacture to that of the
first film and had essentially the same properties with two
exceptions. The tie layer which was adjacent to the heat sealable
exterior layer and made up about 31.4 wt % of the film structure
was a copolymer of ethylene modified with maleic anhydride,
contaminated with about 30 wt % of a copolymer of 1-butene to
render this layer frangible and cause the film to have a reduced
initial peak peel strength of 2700 g/inch determined in accordance
with ASTM F88 when heat sealed at 300.degree. F. for 1 second at 30
psi. In addition, the heat sealable exterior layer was a mixture of
the same VLDPE (density of 0.910 g/cm.sup.3 and melt index 0.6) as
used in the first film and ethylene vinyl acetate (EVA) with a
vinyl acetate content of 5%, a density of 0.927 g/cm.sup.3 and a
melt index of 1.5 g/10 min.
The second film was then heat sealed to the first film with seal
bars which followed the outline of the open mouths of the cavities
but were spaced outward from the mouths so as to act on a perimeter
of flat first film which extended out from the mouths 75 mm. The
seal bars each had a double bead profile with the inner bead being
spaced about 2.5 mm inside the outer bead. Each of the corners of
the molds had a 30 mm radius and so the seal bars had similar
radii. This left a roughly triangular shaped area extending out
from the heat seal at each corner which fell within the rectangle
defined by the sides and edges of each mold (The apex of each
triangle being a corner of this rectangle and the base being the
radius in the heat seal at that corner). Heat was brought to bear
on one of these triangular areas for each cavity causing it to heat
set. The seal bars were heated to 135.degree. C. and pressed into
the film with a pressure of 5 bars. The heat set areas were heat
set using about the same temperature. Before the heat seal and heat
set operations, a small piece of Teflon tape was inserted between
the first and second film in each of the areas which were to be
heat set to inhibit them from adhering to each other as a result of
the heat set treatment. A small chevron was cut out of each piece
of tape so that its apex faced away from the cavity and its base
would be immediately adjacent to the heat seal upon the formation
of the heat seal. Upon heat sealing and heat setting a chevron
shaped feature was formed in which the first and second films were
adhered together. The first and second films lightly tacked
together in the triangular areas, probably due to imperfect
placement of the Teflon tape (Some portion of the first and second
films were probably permitted to directly contact in these areas)
but they were easily separated to provide two graspable tabs for
each het set area.
A rectangle was then cut out of the webs of the first and second
films around each sealed package, which rectangle had sides
adjacent to the outer heat seal except where the heat seal had a
radius. This yielded isolated packages, each of which had a
triangular piece of separate first and second film at each corner
projecting out from the heat seal in the plane of the heat
seal.
Each isolated sealed package was then subjected to a heat shrink
procedure. Each isolated package was carried by a conveyor belt
into a heat shrink tunnel where it was subjected to between
85.degree. C. and 95.degree. C. for between 3 and 5 seconds. This
caused the first and second films to shrink against and for fit to
the contained 32 ounce turkey ham. The triangular pieces which had
not been heat set shrunk away so as not be readily apparent.
However, the heat set triangular pieces remained, with each
providing two readily graspable tabs after fracturing the light
tacking between them which is described above.
The process was repeated a number of times to yield a number of
sealed heat shrunk packages.
The sealed heat shrunk packages were still hermetically sealed but
in a number of cases it was apparent that the inner heat seal had
been opened by the stresses which arose as the first and second
film shrunk against the contents of the package.
The two tabs of a given heat shrunk sealed package were grasped and
pulled away from each other in a direction generally perpendicular
to the plane of the major surface of the tabs. The heat seals of
the package were readily peeled apart just using a comfortable
amount of manual force, thus opening the package and providing easy
access to its contents. The fracture occurred cleanly adjacent to
the heat seal layer of the second film so the package was opened
without skinning or webbing. The same result was obtained for a
number of packages.
Example 2
Example 1 was essentially repeated with the following exceptions.
The second film in this example had its exterior heat sealable
layer contaminated with 25 wt % of the same butane-1 contaminate as
was used in the adjacent layer of the second film in Example 1 and
this heat sealable layer was based on the mixture of VLDPE and
mLLDPE as was used in the heat sealable layer of the first film and
did not include any EVA. The layer adjacent to this layer was not
contaminated and had the same make up as this adjacent layer in the
first film. The peel strength of this film in a standard test was
3300 g/inch.
The areas of the first film which would later form one of the
graspable tabs on each of the sealed packages were covered with
magic marker to inhibit its adhering to the corresponding portions
of the second film. This was entirely successful in preventing such
adhesion. This meant, however, that there was no stress
concentrator at the juncture between the graspable tabs and the
first heat seal.
A series of heat seal temperatures from 125.degree. C. to
155.degree. C. in 10 increments were used and of the four only the
155.degree. C. heat seal temperature failed to give a heat seal
which could be easily peeled.
Example 3
Example 1 was essentially repeated except that the areas of the
first film which would later form one of the graspable tabs on each
of the sealed packages were covered with ink in a first run and
with a label in a second run. In both runs adhesion to the
corresponding portions of the second film was entirely
prevented.
The above disclosure is for the purpose of illustrating the present
invention and should not be interpreted as limiting the present
invention to the particular embodiments described but rather the
scope of the present invention should only be limited by the claims
which follow and should include those modifications of what is
described which would be readily apparent to one skilled in the
art.
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